The global interest in a hydrogen economy has been stimulated by the promise of clean energy production using hydrogen in fuel cells. A reduction in CO2 emissions, however, will require sustainable hydrogen production based on renewable energy using solar, wind and biomass sources. Currently about half of all the hydrogen produced is derived from natural gas, with the balance produced primarily using other fossil fuels, including heavy oils, naphtha and coal. Only 4% is generated from water using electricity derived from a variety of sources (1-3).
Hydrogen can be produced from certain forms of biomass by biological fermentation (4), but yields are low. The maximum hydrogen production from fermentation, assuming only acetate or butyrate is produced from glucose, isC6H12O6+2H2O→4H2+2CO2+2C2H4O2  (1)C6H12O6→2H2+2CO2+C4H8O2  (2)
Four mol-H2/mol-glucose could be obtained if only acetate is produced, but only 2 mol/mol if butyrate is the sole end product. Current fermentation techniques produce a maximum of 2-3 mol-H2/mol-glucose. Thus, most of the remaining organic matter is essentially wasted as a mixture of primarily acetic and butyric acids, despite a stoichiometric potential of 12 mol-H2/mol-glucose (1). The greatest hydrogen yield theoretically possible using microorganisms (without an external source of energy) is therefore 4 mol-H2/mol-glucose based on production of acetic acid. Higher yields can be achieved using a photobiological process and supplemental light, or using pure enzymes, but neither of these methods so far show promise for economical production of hydrogen (5-7).
Thus, there is a continuing need for improved methods and apparatus for hydrogen production.